Why isn't the achievement unlocked of the skull blind in Halo 3?

[Fullnaruto11]

I'm passing the story on Heroic on Halo 3. I want to collect all skulls. I started of with the first one which is Skull: Blind. I got it and nothing happened. So I passed it further on, after a while I got more skulls. I checked my achievements and there was only 3, Iron, Black Eye, and Though Luck. Why isn't Blind in there?
if you want to play my gamertag is Fullnaruto3

 

[Big Btom]

it's a silver skull and won't give you an achievement. whats your tag? I play halo 3 also

 

[peri]

Only gold skulls will give you achievements because gold skulls are difficulty multipliers. The silver skulls won't give you achievements because they aren't difficulty multipliers. They are just fun to have on, like Grunt Birthday Party or IWHBYD. You can find out what color the skull is by hitting X on the campaign screen.